The invention relates to parts for process valves, provided with a thermally sprayed coating to protect them from wear. The process valves are components of larger technical apparatuses used in the chemical and petrochemical industry, in the mining industry and in other processing industries.
The invention is thus particularly advantageous when used in these fields.
It is important that the process valves do not have any or have extremely few leakages, and that no deposits occur under any operating conditions between the waiting intervals. This requires good adhesive wear resistance from the sealing faces of the valves.
The valve surfaces in contact with the process media (e.g. liquids, sludge and solids) are generally exposed to aggressive, particularly corrosive effects of the media. The high rate of the process media also causes erosive and/or abrasive wear. Further, a prerequisite for the control and reliability of all adjusting motions is that the sealing faces of the valves, which rub against each other, must have a low friction coefficient.
Coating is an effective way of achieving the required low friction coefficients on the surfaces of the parts for process valve and simultaneously reliably protecting the surfaces from wear and corrosion. Different coating techniques, such as hard chrome plating, non-conductive nickel plating, coating with stellite using powder plasma dot welding, thermal spraying and sintering of free-flowing alloys, such as NiCrBSi, are used to protect the surfaces of the valve parts. Thin electrolytically or non-conductively applied coatings tend to wear under heavy load (e.g. high pressure exerted on the surface) and have an operating temperature not higher than 400.degree. C. In the powder plasma dot welding and the sintering of the free-flowing alloys, the process valve parts, subjected to high thermal load during the coating process, tend to distort. Further, the sintering of the coatings in the furnace is time consuming and expensive.
The thermal spraying methods, such as plasma spraying, detonation spraying and high-rate flame spraying (HVOF), are the most effective methods of applying thick and wear resistant coatings that are able to resist high surface pressure even at high temperatures and at the same time, exhibit good wear resistance. Different thermally sprayed coating systems are used to coat process valve parts. Such coatings are used in particular on the bearing and sealing faces of the valves. The function of the coatings is to reduce adhesive wear, generally together with abrasive wear, and to reduce the friction coefficients of the surfaces rubbed against each other. In addition, high moist corrosion and temperature stability are simultaneously required in many cases.